The invention relates to a sample holder for a cryostat insert, more especially but not exclusively to a sample holder for a top-loading cryostat insert.
Conventionally sample holders for cryostat inserts are constructed by brazing and/or welding tubes, plates and disc-shaped heat shields onto a vacuum flange. One type of cryostat in widespread use is the top-loading type in which the sample holder fits into a cryostat insert tube having a flange at one end for mating with a corresponding flange of the sample holder and being closed at the other end. When the insert tube and sample holder are fitted together to form the insert, the space inside the insert can be evacuated and the insert loaded into the top side of a top-loading cryostat. Cryostats of this kind are designed to receive an insert of a fixed diameter, for example one-and-a-half inches, two inches and 50 mm. For magnet cryostats the diameter is usually dictated by the superconducting magnet coil dimensions, i.e. the inner coil diameter for a conventional vertical bore magnet coil alignment.
Top-loading cryostats with inserts of this kind are generally not considered to be ideally suited to perform experimentally demanding optical measurements. However, it is common to perform experimentally simple optical measurements, such as photoluminescence, in top-loading cryostats with the aid of optical fibers. The optical signals are conveyed to and from the sample via an optical fiber which extends into the insert via a vacuum-tight feed-through on the sample holder flange and to the close proximity of the sample, which is mounted near the base of the sample holder. For optical measurements of greater experimental complexity, optical fiber based excitation and signal collection is often not convenient in which case cryostats with side windows are generally favored so that free-space optics may be used. In the case of magnet cryostats, a split-coil magnet alignment is required if side window access is desired. Split-coil magnet cryostats are several times costlier than equivalent vertical-coil magnet cryostats. In a vertical-coil magnet cryostat a single base window is often provided for limited external optical access.
According to the invention there is provided a sample holder apparatus based on a conventional cage assembly optical rail system which extends at least on the vacuum-side of a sample holder vacuum flange. The cage assembly system preferably extends also on the air-side of the vacuum flange to form a contiguous cage assembly system extending on both sides of the vacuum flange which is provided with a window for optical communication through the vacuum flange. A cage assembly based sample holder complete with free-space optical components can be sleeved into a cryostat insert tube thus to allow a wide range of optical measurements to be performed at low temperature.
In the preferred embodiment, the sample holder flange is based on a standard vacuum flange into which has been bored on the vacuum side a group of four blind bores conforming to the square grid of a conventional cage assembly system and for receiving cage assembly rods. Rod holders may be implemented in many different ways other than blind bores, for example as sleeves extending from the main body of the vacuum flange. Preferably, a corresponding group of blind bores, conforming to the same square grid, is provided on the air-side of the flange for receiving further cage assembly rods, the bores on the air-side and vacuum-side of the flange being arranged in co-axial pairs with one of each pair on either side of the flange.
In the preferred embodiment, the rods used on the vacuum side are thin-walled stainless steel tubes, rather than the solid rods of conventional cage assembly system, thus providing a much lower thermal mass than solid rods. The interior of the tubes can also provide shielded routing channels for electrical leads or optical fibers which may pass into the tube interior via side holes in the tube wall. Conventional square cage plates are machined down at their corners to fit within an insert tube of inner diameter 49.6 mm so as to conform to an arcuate profile of a single circle, the center of which lies on or close to the main optical axis of the cage assembly system.
By basing the sample holder on a cage assembly system, a sample holder having the flexibility of a cage assembly system can be provided. Optical components such as lenses, irises, filters and polarizers can be moved, added and removed at will. Double cage plates can be incorporated to allow the cage assembly to be split up into detachable modules. For example, the lowest module, which may be a sample mounting module, can be detached and later reattached and realigned. A module, such as an optics module, may then be positioned above the sample mounting module. The optics module may be exchanged for another optics module for performing a different kind of optical measurement. If the cage assembly system is extended out onto the air-side of the flange, a camera such as a CCD camera can be mounted on a cage plate so as to view into the insert tube through the window provided in the vacuum flange. Other optical detector devices could also be mounted on the air-side, e.g. photomultipliers, CCD array detectors, multichannel plates and so forth.
Cage assembly systems are well known for bench-top optical arrangements.
One commercially available system is from the U.S. company, Thor Labs, Inc., New Jersey. This system is based on a square grid of four parallel rods of diameter 6 mm, the rod axes lying on a square of 30 mm side length. Along the rods are mounted cage plates having a corresponding square grid of four bores through which the rods can pass. The bores are arranged in respective corner regions of the plates which have outer dimensions of 40.6 mm square. The plates may have threaded holes of a standard diameter of 1.035 inches, i.e. approximately 25 mm, and standard threads such as RMS mount threads and C-mount optics threads. Optical components can then be mounted in these threaded holes.
Another commercially available cage assembly system, which is somewhat smaller, is from the German company Spindler and Hoyer and has the trade name xe2x80x9cMikrobankxe2x80x9dand is based on a square grid of 16 mm side length. The exterior side lengths of the cage plates are 25 mm and the threaded through holes are 16 mm in diameter for receiving 15 mm diameter optical components. The diameter of the rods is 4 mm.
A third commercially available cage assembly system is from the U.S. company, AF Optical Company of Irvine, Calif. and has the trade name xe2x80x9cMICROPTICxe2x80x9d. This system is based on annular cage plates which are 10 mm thick and have an outer diameter of 49.25 mm and an inner diameter of either 25 mm or 30 mm. Each cage plate is provided with four holes based on a square grid dimensioned to receive rods of 6 mm diameter.
Cage assembly systems based on polygonal grids other than square ones could also be used, for example triangular, rectangular, pentagonal or hexagonal grids. Moreover, the number of rods provided need not be equal to the side number of the polygon. For example, with the square grid three rods can be used instead of four. This provides open access to one side of the cage assembly so that optical components can be inserted into and removed from a cage plate laterally with ease. For example, a polarizer may be inserted into a cage plate and then removed when no longer required. The cage plate can be left for possible future use. In addition a system using three rods is perfectly defined, being neither overdefined, i.e. hyperstatic, nor underdefined, i.e. hypostatic.
Further aspects of the invention are exemplified by the attached claims.